function insert(str,index,insert_str)
    return string.sub(str,1,index-1) .. insert_str .. string.sub(str,index)
end
print(insert("hello world",1 ,"start: "))
print(insert("hello world",7,"small:"))
function remove(str,start_index,end_index)
    return string.sub(str,1,start_index -1 ) .. string.sub(str,start_index + end_index)
end
print(remove("hello world",7,4))
function ispali(str)
    return str == string.reverse(str)
end
print(ispali("12321"))
print(ispali("12322"))